PEG-maleimide labelling can be used to determine how many cysteine residues of a protein are exposed to the surface and can be expected to react with maleimide conjugated dyes. The advantage of labelling with PEG is that it causes a large change in the apparent molecular mass of the protein which can be detected by SDS-PAGE.
Make a stock of PEG-maleimide in DMSO (a 1mM stock is easily possible)
Ensure that there is no reducing agent in your protein buffer (desalt if necessary)
Mix protein and PEG together in molar ratio of 1:10. Ensure that the final [DMSO] is low (eg 1%).
Take aliquots from the reaction throughout a timecourse and quench the reaction in each aliquot with DTT. A first-guess timecourse should run from 30 s to 15 min
Run a gel of the aliquots - a ladder should appear as the bands shift. Running a timecourse should enable easy quantification of the number of sites available as intermediate steps are visible on the ladder.